查看更多>>摘要：? 2022 Elsevier B.V.Undrained shear behavior of loess after the K0 consolidation was investigated through laboratory experiments. Both the monotonic and cyclic triaxial tests were performed on the intact loess samples retrieved from a construction site in Yan'an city, China. It was found that the coefficient of lateral earth pressure at rest (K0) of the intact loess is stress-dependent, and it varies between 0.46 and 0.85. Under the monotonic loading condition, the peak q in the undrained shear test is higher for the samples under the K0 consolidation than those under the isotropic consolidation. Besides, the cyclic loading tests were conducted on the loess samples subjected to different magnitudes of the initial deviatoric stress, including the K0 stress state. The test results revealed that different initial stress conditions cause distinctly different failure patterns. It includes the cyclic mobility with progressive development of the axial strain, and limited deformation followed by an abrupt gain of the axial strain with no precursor. In accordance to the build-up of the excess pore water pressure during the tests, each failure pattern of loess was interpreted in the q-p' diagram alongside the undrained instability state (UIS) line. The outcomes of this paper suggest a pressing need to thoroughly consider the K0 values of the intact loess with variations of the initial stress state.
查看更多>>摘要：? 2022 Elsevier B.V.Microbially induced calcite precipitation (MICP) has received greater attention of geotechnical engineers recently as a potentially effective and sustainable approach for biocementation of sands. Present study investigates spatial variability and synergistic effects of bacterial hybrids of Sporosarcina (S.) pasteurii and Bacillus (B.) sphaericus for sand biocementation which is important to explore to stimulate real field conditions where multiple urease producing can be present in soil environment. Large-scale testing was conducted in rectangular tank (135 cm × 113 cm × 65 cm) filled with Narmada sand (1.21 Ton), treated using bio-chem distribution system consisting of bioreactor, chemical mixer, circulation tank, injection pipes, treatment tank, and drainage outlets. The design of bioreactor and slotted injection pipes are the major novel aspect of this study for on-site bacterial cultivation and uniform calcite precipitation throughout the treatment depth to increase liquefaction resistance of sand which was not investigated in previous large-scale testing studies. Bacterial hybrid was first cultivated, then inoculum of 10 L was prepared, and finally, 200 L of bacterial solution was cultivated in bioreactor. Sand was bioaugmented with hybrid bacteria in tank, subsequently injected 0.50 M urea and cementation solution periodically up to 8 days. Biosparging was also implemented for 10–12 h daily using air pumps to ensure aerobic conditions. During the treatment, pH, electrical conductivity (EC), and total dissolved solids (TDS) were measured periodically as indicators of biogeochemical reactions occurring during biotreatment. Dynamic cone penetration tests (DCPTs) were performed and DCPT-blows up to 30 cm depth were measured at 20 locations. Contours of dynamic cone penetration index (DCPI) and calcite precipitation were used to assess spatial variability of biocementation. Average DCPT blow count for untreated sand was 5, but it increased up to 50 with 10.6% calcite content, for 30 cm penetration. The DCPT results showed significant improvement in liquefaction resistance. Biocemented samples were collected from various locations to measure calcite content and conduct SEM. Overall, the large-scale testing showed effective and substantial biocementation of sand using the cultivated hybrid bacteria and bio-chem distribution/air sparging system developed in this study. The spacing of injection points and the amounts of injection biotreatment solutions can be optimized to achieve uniform, adequate, and cost-effective biotreatment under actual field applications.
查看更多>>摘要：? 2022 Elsevier B.V.This work describes field evidence of gravity-induced cracking that has been identified on the failure scar of a small rockslide (volume = 6–7 m3) that occurred in March 2004 in the Rosandra Valley near the town of Trieste (north-eastern Italy). This shallow rock slope failure involved a limestone slope that was re-profiled through blasting about 135 years ago for the construction of an old railway, which has now been reconverted into a cycle path. Field observations ascertained that the slope failure was characterised by a cascading rupture process in which adjacent blocks collapsed in rapid sequence as a result of the progressive breakage of a number of rock bridges (domino-like collapse), thus highlighting a progressive failure mechanism. The rock bridges were localised in eccentric positions on lateral and rear release planes and failed in tension as a result of combined tensile and bending loading conditions (tensile strength of intact rock = 5 ± 2 MPa). The rockslide was triggered by cyclic loading related to freeze-thaw (ice jacking) that occurred over some consecutive days of daily snowmelt and nocturnal frost. The recognition of newly formed fractures also proves the current progressive development of 3D rupture surfaces of some unstable blocks susceptible to failure. This study provides innovative content to detect gravity-induced cracking in the field, which is an important precursory sign relating to the progressive failure of rock slopes. Gravity-induced cracks can be distinguished from pre-existing discontinuities for their more irregular profile (zigzag pattern), rougher surface, lower persistence and potentially different orientation. This study also provides some new insights into the step-path failure process of steep rock slopes. The mechanical process of initiation, growing and coalescence of gravity-induced fractures is strongly time- and stress-dependent. In the absence of external changing factors that profoundly modify the acting stress state (for instance, engineering countermeasures), progressive failure is a dynamic and irreversible process. For excavated slopes, the creation of a new rock face may abruptly provide the kinematic freedom of potentially unstable blocks, thus determining a very quick stress redistribution. If the mechanical system is not able to redistribute the stress on resisting stiff parts without overcoming the intact rock strength, gravity-induced cracking initiates and propagates until reaching slope collapse within times that are much shorter (from hours to 100–200 years) compared to natural rock slopes subjected to long-lasting geological processes (from 1000 to 2000 up to 10,000–20,000 years).
查看更多>>摘要：? 2022 Elsevier B.V.The heritage restoration and the design of modern stone masonry structures bear in mind the material response to fire, the residual strength and post-fire resilience. In this attempt, the decomposition of fire-exposed natural and engineered stones and their mineralogical transformations are reported. The fire-induced petrographic change is further correlated to rise in shrinkage and porosity, and loss of mechanical properties, such as strength, toughness and stiffness. Stones of different nature (limestone, sandstone, quartzite, polymer/quartz-based engineered stone) are selected to trigger various thermal degradation mechanisms. Also, the stones were sorted considering their sustainability profile since all stones, except the artificial ones, were collected from local quarries and linked to the regional heritage and architecture. An integrated inspection methodology is designed that implements non-destructive monitoring on mechanical tests. A novel proof-loading concept is introduced at which the residual performance is assessed exclusively based on ultrasound pulse velocity (UPV) and early acoustic emission (AE) data. The damage mechanisms that dominate fracture and the shift of failure from strong brittle to weak pseudo-ductile with an early transition from crack initiation to propagation are determined based on AE features analysis.
查看更多>>摘要：? 2022 Elsevier B.V.Engineering geological characteristics of soils have been developed based on the assumption of two limiting drainage boundary conditions: fully drained and fully undrained. In recent years, an increasing number of research works have been applied to study the effects of partially drained conditions on the behavior of soils. Different strain paths can be employed to analyze soil behavior, including limiting conditions. However, it is well known that the fundamental parameters of sands are dependent on the strain path that they experience; the post cyclic behavior of sand under different strain paths in plane strain conditions is not evaluated appropriately yet. In this paper, using a stack-ring type direct simple shear apparatus, the main characteristic parameters of Firoozkuh No. 161 medium sand are evaluated. Monotonic and post cyclic behavior of sand specimens under dilative, contractive, and constant volume conditions are compared. It is shown that the strain path significantly affects sand behavior. Achieved results demonstrated that slopes of steady-state lines in stress paths (asymptotic stress ratios) are dependent on the magnitude of partial drainage for both monotonic and post cyclic conditions, and the initial stress state of samples at the commence of post cyclic loading is an important parameter to define post cyclic behavior of sands under partially drained situations.
查看更多>>摘要：? 2022 Elsevier B.V.High virgin rock temperature represents one of the greatest bottlenecking constraints for waste tailings recycling in deep mines, and the unique geothermal environment associated with each particular mine calls for customized backfill operation. In this paper, a new fully coupled thermo-hydro-mechanical-chemical model is developed for cemented backfills by invoking basic conservation equations. In addition, the effect of thermal pressurization induced by constrained thermal expansion of pore water that is missing in existing backfill models has been rationally considered in this study based on the generalized Biot's theory. By scrutinizing the complex backfill behavior at various initial and surrounding rock temperatures, novel insights are obtained into the different backfill performance under diverse geothermal conditions. Our numerical investigation has demonstrated that the strong nonlinearity of backfill response stems from the intricate competitions among the multi-physics processes present in the material. In particular, the pore pressure evolution in backfills is shown to be dominated by the interplay among chemical shrinkage, thermal pressurization associated with hydration-heat generation and boundary heating, as well as the resulting fluid migration that modulates the pressure distribution. Moreover, our calculations have also shown for the first time how boundary heating can contribute to backfill instability after an incubation period, highlighting backfill operation in hot mines as the least favorable thermal condition for waste tailings recycling. By examining the effectiveness of different countermeasures in mitigating the impact of boundary heating, guidelines for optimal design are provided in this study to facilitate customized backfill operation under high virgin rock temperatures.
查看更多>>摘要：? 2022 Elsevier B.V.Dry-wet cycle induced loess spalling and collapse frequently occurs in the Loess Plateau, which is an influential trigger for the instability of infrastructure. In order to evaluate the resistance of basalt-fiber reinforced loess to dry-wet action, a digital-image system was used in triaxial shear tests with failure mode and shear strength discussed. The apparent feature and microstructure of fiber-reinforced loess were analyzed using the Particles (Pores) and Cracks Analysis System (PCAS) and Scanning Electron Microscope (SEM). Results show that the shear strength of fiber-reinforced loess gradually decreases with dry-wet cycles, with maximum decline after initial two cycles, while it increases and then decreases at higher fiber contents, with pronounced resistance observed at a fiber content of 0.6%. The loess before reinforcement exhibits brittle failure mode while the plasticity becomes more obvious as the fiber content increases, i.e., overall bulging damage but returns to brittle failure after 5 cycles. The crack ratio, as revealed by PCAS, varies consistently with the strength decay after dry-wet cycles. SEM images indicate that microcracks appear in the loess after dry-wet action and the fiber network enhances the resistance to dry-wet cycles, with the deterioration occurring mainly at the soil-fiber interface. Based on the continuum damage theory, a statistical damage model was proposed for fiber-reinforced loess considering dry-wet and loading damage. Error analysis results show that the model well describes the stress-strain behavior of reinforced loess with different fiber contents after dry-wet cycles.
查看更多>>摘要：? 2022Fractures and their connectivity are essential for fluid flow in low permeability formations. Geological outcrops can only provide two-dimensional (2D) information, but subsurface fractures are three-dimensional (3D). The percolation status of 3D fracture networks and their 2D cross-section maps are rarely investigated simultaneously. In this work, we construct 3D fracture networks with their geometries characterized by different stochastic distributions. Cross-section maps are taken to mimic real outcrops, and clusters are labeled to check the percolation status of 3D fracture networks and their 2D cross-section maps. The properties, reflecting the connectivity of two essential phases, are summarized and analyzed. We find that clustering effects impact local intersections significantly but have negligible impacts on fracture intensities of 3D fracture networks. The number of intersections per fracture is not a proper percolation parameter for complex 2D and 3D fracture networks. Real fracture networks in the subsurface should be geometrically well-connected and over-percolative if their outcrop maps have a spanning cluster formed. Empirical limits are provided to predict the fracture intensity and connectivity of subsurface 3D fracture networks based on their outcrop maps. If a spanning cluster is formed in the outcrop map, the corresponding 3D fracture networks should be over-percolative, and its fracture intensity can be larger than 3.5 times the intensity at percolation. However, if no spanning cluster is formed in the outcrop map, but the fracture intensity is larger than 0.43 times the intensity at percolation, the corresponding 3D fracture networks can form a spanning cluster and show good connectivity.
查看更多>>摘要：? 2022 Elsevier B.V.Desiccation crack is an important factor affecting engineering properties of clayey soils. In this study, the effects of compaction state and wetting-drying cycles on desiccation cracking behaviour of a clayey soil were investigated. Nine samples with various compaction water contents and dry densities were subjected to three wetting-drying cycles and the evolution of desiccation cracks was quantitatively described. The results reveal that both compaction water content and dry density play an important role in the development of desiccation cracks. Specifically, for the samples compacted on the dry side, desiccation cracks develop simultaneously. This phenomenon is also observed in samples compacted on the wet side with low densities. However, for the samples compacted on the wet side with high dry densities, desiccation cracks develop sequentially. Moreover, the higher the compaction water content, the faster development of desiccation cracks, and the smaller surface crack ratio, total crack length and average crack width. The higher the dry density, the slower development of desiccation cracks, and the smaller surface crack ratio and total crack length. Furthermore, the development of desiccation cracks can be divided into two stages according to the evolution of crack propagation velocity: (a) crack length extension, where total crack length increases rapidly with little changes in average crack width, and (b) crack width widening, where total crack length no longer changes and average crack width increases until it stabilizes. In addition, surface crack ratio, total crack length and average crack width all show an upward trend as wetting-drying cycles progresses. This tendency is more pronounced in the soil sample with a higher compaction water content and a larger dry density.
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